Abstract
The pre-2004 9.0 Aceh earthquake Global Positioning System (GPS) velocity field along the western margin of the Sunda plate was dominated by the long-term secular velocity and elastic strain. Since then a sequence of the great earthquakes includes the 2005 8.6 Nias, 2007 8.5 Bengkulu, and 2012 8.6 and 8.2 Wharton Basin earthquakes, have occurred in different segments of the subduction zone and its vicinity, which resulted in significant coseismic and postseismic deformation on the Sunda plate. This study combined the published and the estimated GPS velocity fields between 1991–2016 from more than 150 GPS sites. These velocity fields are inverted to examine the angular velocities of the elastic crustal blocks and the variability of the coupling on the subduction trench. This analysis reveals the characteristic of the Sunda subduction interface over multiple earthquake cycles along different segments of the trench, whereby the subduction interface coupling coefficient changed both spatially and temporally after each rupture. The strongly coupled subduction interface along the plate convergence before 2004 earthquake is now partially coupled to freely slipping in the segments that ruptured during the 2005 and 2007 earthquakes, according to the present-day GPS velocity field (2012.2–2016.0). Interestingly, the best fitting model shows that the Siberut segment (0.5–2.0°S) remained fully coupled throughout the years. The result implies that the level of coupling along the highly segmented Sunda subduction interface varies over time, and that the great earthquake rupture was likely to be a result of the variation in the coupling.